1. Field of the Invention
The present invention relates to an optical disc apparatus which carries out recording or reproducing of information by the tracking of an optical head with respect to a rotating optical disc and, more particularly, to an eccentricity correction apparatus for offsetting the adverse effects of eccentricity in the tracks of the optical disc.
2. Description of the Prior Art
In general, in an optical disc apparatus of the above kind, the optical disc has concentric circular or spiral recording tracks as the information recording medium, and the recording or reproducing of information for the recording track of the optical disc is carried out by linearly moving the recording head in the radial direction of the optical disc while rotating the disc.
In such an optical disc apparatus, the optical head must precisely follow the recording tracks of the rotating disc. For this reason, tracking control is ordinarily carried out using a tracking servo system.
In the optical disc apparatus of the above description, when the optical head is moved from an arbitrary recording track which is in rotation to another recording track, tracking is interrupted for a while to move the optical head for a distance which corresponds to the separation between the present and desired tracks, and tracking is then resumed. However, the recording tracks themselves of the optical disc are generally not necessarily concentric with respect to the center of the rotation shaft of the motor that drives to rotate the optical disc, and are often eccentric. Because of this, when tracking is interrupted once, the relative position of the optical head with respect to the recording track has a dispersion that corresponds to the eccentricity amount so that, when tracking is resumed after the head has been moved, it leads to a drawback that accuracy tends to be lowered by the deviation corresponding to the dispersion. This results in difficulty in reducing the time for positioning the optical head to the desired track.
Further, in the optical disc 1 as shown in FIG. 1, a spiral pre-groove 2 is provided on the surface of the substrate for tracking, in order to facilitate recording and reproducing. Since the recording track is provided in the pre-groove 2, the pre-groove 2 is referred to simply as the track hereinafter.
In FIG. 1, for facilitating understanding, the track pitch is drawn schematically with wide separation. However, the real track is formed with a relatively small pitch on the order of microns (.mu.m).
The optical disc 1 is placed on a spindle motor, as will be described later, to be rotated in the direction of the arrow R of FIG. 1. Accompanying the rotation, the beam spot (laser spot) of the optical beam projected from the optical head is traced on the track 2. The beam spot is shifted outward by one track for each one rotation of the optical disc 1.
Recording of information is made by the change in reflectivity generated by the phase transition or the like in a thin film layer in the track portion of the disc, through irradiation of the beam spot whose intensity is modulated by an information signal.
FIG. 2 shows a track control device for causing the optical head 4 to follow the track 2 in recording and reproducing information from the optical disc 1 as described above. In FIG. 2, a light source is omitted to simplify the track control device.
In the optical head 4, reference numeral 5 is an objective lens which projects the beam spot of the light beam on the optical disc 1; 6 is a driving coil for controlling the position of the objective lens 5 in the radial direction of the optical disc 1; 13a and 13b are photodetectors for detecting the intensities of reflected light rays l1 and l2, respectively, photodetector 13a being connected to the negative input terminal of a differential amplifier 15 via a sensor amplifier 14a while photodetector 13b is connected to the positive input terminal of the differential amplifier 15 via another sensor amplifier 14b.
It should be noted that an optical system is provided for projecting a beam from the laser diode as will be described later. However, for the sake of convenience of explanation, the beam incidence optical system is omitted in FIG. 2 and the configuration of the reflection optical system is principally illustrated.
If the beam spot is projected correctly on the pre-groove 2 as shown in the figure and tracking is carried out precisely, then the intensities of both reflected lights l1 and l2 become equal and the track difference signal voltage TS output from the differential amplifier 15 becomes zero.
If the beam spot is shifted toward the center of the optical disc 1 as shown by the arrow in FIG. 2, the reflected light ray l1 is reflected from the surface portion of the disc (the area outside the pre-groove 2) with greater intensity, giving a larger detected value at the photodetector 13a, and causing the differential amplifier 15 to output a negative track deviation signal voltage -TS.
If the beam spot is shifted in the direction opposite the arrow in FIG. 2, the reflected light ray l2 has a greater intensity but opposite to the above, and a positive track deviation signal voltage +TS is output from the differential amplifier.
The track control is carried out by applying the positive and negative track deviation signal voltages .+-.TS to the driving coil 6 and controlling the position of the objective 5 to drive the track difference signal voltage TS to zero. By applying the tracking servo to make the track deviation signal voltage TS zero, the beam spot follows the track 2 to carry out correct recording and reproducing.
The optical disc apparatus can carry out the operation of repeatedly reproducing an identical track of one turn, the so-called track-on mode, in addition to the ordinary operation of recording and reproducing as described above. The track-on mode operation can be realized by applying a jump pulse to the driving coil 6 each time the optical disc 1 completes one rotation, to cause the beam spot to jump to the adjacent previous track. In carrying out the track jump, the loop of the tracking servo is open.
The solid waveform in FIG. 3 is the output waveform of the track deviation signal voltage TS when a track jump is carried out by passing a current in the driving coil in the direction of the arrow in FIG. 2, and the broken-line waveform is the output waveform for the track deviation signal voltage TS when the track jump is carried our in the opposite direction.
The track deviation signal voltage TS is output as a waveform for one cycle every time the optical head 2 traverses the pre-groove 2. Therefore, the number of waves in the track deviation signal voltage TS corresponds to the number of tracks jumped.
When the optical disc 1 is mounted on the spindle motor, the central point Cp1 of the optical disc 1 and the center Cp2 of the rotation shaft of the spindle motor typically do not coincide perfectly, producing eccentricity for the optical disc 1 as shown in FIG. 4. This can be due to the accuracy of the center hole of the optical disc 1, errors generated in chucking the disc to the spindle motor, and so on.
To illustrate the situation schematically, let us approximate the track by concentric circles 2a and 2b as shown in FIG. 4. If one assume that the optical disc 1 is rotated in the direction of arrow R and the beam spot from the optical head 4 is projected on a fixed position, then the locus of the beam spot will be a dotted circle which has the rotation center Cp2 of the spindle motor as the center. Thus, for the track 2a, the beam spot coincides with the track 2a at points a and c, but deviates by the amount of eccentricity at points b and d, i.e., it is shifted to the outside of the track 2a at point b while it is shifted to the inside of the track 2a at point d.
FIG. 5 shows the waveform of the track difference signal voltage TS on which each of the points a, b, c and d is indicated.
If the tracking servo is activated, the track deviation signal voltage TS is applied to the driving coil 6, and the beam spot is corrected to follow the track 2 so as to be moved to the inside at point b and to the outside at point d.
As in the above, even if the optical disc 1 is placed on the spindle motor with eccentricity relative to the rotating center of the motor, when the tracking servo is turned on, the beam spot can be made to follow the track 2 correctly. However, when a track jump is carried out, the loop of the tracking servo is brought to an open state. Because of this, when the beam spot which has been following the track 2a is jumped, for example, to the track 2b, the beam spot deviates from the track 2a in the vicinities of the points b and d. Therefore, a problem arises in making a jump from the track 2a to the track 2b in that the track jumping is unstable by the variations in the jumping distance in the vicinities of the point b and the point d.
A successive comparison type A/D converter is known for use as an A/D converter in the prior art optical disc apparatus which carries out high precision A/D conversion. The A/D converter has a voltage comparison circuit, a successive comparison register, a D/A converter that constitutes the feedback circuit, a clock oscillator which carries out the overall control, and so on.
With this converter, a digital quantity which corresponds to the input analog voltage is arranged to be output in the following way. Starting with the least significant bit of the successive comparison register, a converted voltage that corresponds to each bit is arranged to be output from the D/A converter to compare the converted voltage with the input analog voltage in the voltage comparison circuit. If these two voltages do not coincide, the same procedure is successively repeated for each bit until agreement is obtained.
However, in the successive comparison type A/D converter as noted above, an analog voltage is converted to a digital quantity by the successive comparison of the smallest bit unit. This type of device has generally been unsatisfactory because of the inability for high speed operation due to a relatively long time required to obtain the digital quantity.
A parallel comparison type A/D converter is also known as an existing A/D converter which can carry out A/D conversion at high speed. The A/D converter comprises a resistor ladder circuit which generates (2.sup.n -1) reference voltages by subdividing a given voltage into 2.sup.n equal parts when the digital quantity to be output has n bits, (2.sup.n -1) voltage comparators to each of their reference terminals for setting each of the reference voltages generated in the resistor ladder circuit, and a decoder which transmits the outputs of the voltage comparators as an n-bit digital quantity after binary coding conversion.
With this construction, an input analog voltage is input parallel to the input terminals of all of the voltage comparators and compared at once with all of the reference voltages, to output from the decoder a digital quantity that corresponds to the input analog voltage.
However, in the parallel comparison type A/D converter described above, a large number (2.sup.n -1) of voltage comparators are required for the digital quantity of n bits. Because of this, factors for accuracy deterioration such as the dispersion in the threshold voltage, input bias current, input offset voltage, and delay time, for each of the voltage comparators, and the relative accuracy of the resistor ladder circuit, are augmented, giving rise to difficulty in achieving a high accuracy A/D conversion.